Radio-frequency detectors, which include devices that can sense a voltage level, current level, or power level of a radio-frequency (RF) signal, are used in radio applications for a variety of purposes. For example, an RF detector may be used to measure output power from a transmitter power amplifier, or to estimate the signal strength of a received signal.
A simple envelope detector might comprise just a diode and capacitor, or a four diode bridge rectifier. A diode used for power detection applications is typically a junction diode (such as a Schottky diode), fabricated with standard CMOS process technology. These junction diodes exhibit large forward voltage drops and poorly controlled operating parameters, limiting the dynamic range and accuracy of diode-based detectors.
RF detectors may also be constructed by exploiting the quadratic operating characteristic of a transistor, such as a MOSFET. One approach exploits the quadratic relationship between input voltage and output current of a single softly saturated FET, followed by a low-pass filter. Power detectors based on softly saturated amplifiers have a broader dynamic range than a simple diode-based detector, but consume more area. These power detectors also have a relatively low upper frequency limit.
Another well-known approach is to use an unbalanced pair of transistors as a rectifying signal detector. Unbalanced-pair power detectors are less sensitive to temperature than single-diode or single-transistor designs, but have a fundamental built in DC-offset voltage that limits dynamic range. To improve dynamic range, received-signal-strength indicator (RSSI) circuits often use unbalanced transistor pairs coupled to a series of limiting amplifiers. Other circuits extend the dynamic range of a detector by using a variable-gain amplifier, with feed-back loops, to amplify the RF signal so that it falls within the limited dynamic range of the detector. Of course, either of these approaches to improving dynamic range requires larger and more complicated circuits, and more demanding attention to design details, especially as the desired operating frequency approaches the cut-off frequency for the available semiconductor technology.
Simple current- or voltage-rectifier detector circuits are generally followed by a filter, to provide a smooth DC (or low-frequency) output. The output of such a filter is proportional to the average value of the AC input signal. For waveforms of a known shape, such as a sine wave, this output provides all the information that is needed, since the relationship between the average value and other parameters, such as peak value or root-mean-square (RMS) value is known. However, in some applications a direct measurement of RMS power may be desired, even for complex waveforms where the relationship between the average level and RMS level is unknown. Some power detector circuits that rely on the quadratic characteristic of a transistor enable RMS detection. For example, the unbalanced-pair detector discussed above effectively performs a squaring operation, based on a sensed input voltage, along with a rectification operation, thus yielding an output proportional to the power of the input signal. An RMS value may be obtained by taking the square root of the average power. Detectors that yield an output proportional to the RMS level of the input signal are known as true RMS detectors.